As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material may also comprise a layer of gas- and light-barrier material, e.g. an aluminium foil or an ethyl vinyl alcohol (EVOH) foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating. The web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which may be fed to a folding unit to form the finished packages.
More specifically, the pillow packs substantially comprise a main portion, and opposite top and bottom end portions tapering from the main portions towards respective top and bottom sealing bands which extends substantially orthogonal to the axis of the pack. In detail, each end portion is defined by a pair of respective trapezoidal walls which extend between main portion of the pack and the relative sealing fin.
Each pillow pack also comprises, for each top and bottom end portion, an elongated substantially rectangular fin projecting from respective sealing band; and a pair of substantially triangular flaps projecting from opposite sides of relative end portion and defined by respective trapezoidal walls.
The end portions are pressed towards each other by the folding unit to form flat opposite end walls of the pack, while at the same time folding the flaps of the top portion onto respective lateral walls of the main portion and the flaps of the bottom portion onto the bottom sealing band.
Packaging machines for producing packages of the above type are known, typically comprising:                an in-feed conveyor;        a folding unit receiving the pillow packs from the in-feed conveyor and adapted to fold these pillow packs to form the parallelepiped-shaped packages;        a transfer unit for transferring and up-ending sealed folded packages, which is arranged downstream from the folding unit and receives the sealed packages from the folding unit; and        an out-feed conveyor which receives folded packages from the transfer unit and moves them away from the packaging machine.        
Folding units are known, for example from EP-A-2284084 in the name of the same Applicant, which typically comprise:                a chain conveyor for feeding packs continuously along a forming path from a supply station to an output station; and        first and second folding means, which cooperate cyclically, which each pack to flatten respective end portions of each pack and so fold respective fins onto end portions.        
In detail, the first folding means comprise a fixed guide member, which is positioned facing and at a distance from a conveying portion of the chain, and converge towards this conveying portion. The fixed guide member cooperates with bottom end portion of each pack to press it down flat towards the chain.
The second folding means comprise:                a plurality of movable plates hinged to relative links of the chain about relative axes crosswise to the forming path; and        a plurality of cams carried by the conveyor chain and each cooperating, in use, with a relative movable plate.        
In detail, each plate defines a surface adapted to cooperate with a top fin of relative pack. Upon impact with fin, each plate moves from a rest position to a first operative position in which relative surface defines a first angle of over 90 degrees with the axis of the relative pack.
As moving along forming path, each plate moves towards a second operative position in which the relative surface defines a second angle lower than the first angle with the axis of relative package. Accordingly, a central portion of the top fin is folded towards main portion of relative pack.
The second folding means comprise a pair of wheels supported by a fixed structure of folding unit, and a pair of rails converging towards guide member. Each rail comprises a relative first portion arranged below respective wheels and a relative second portion. In detail, the second portions are arranged downstream from first portions, proceeding according to the advancing direction of packs along forming path.
The wheels and the first portions of relative rails define respective passages through which lateral zones of the top fin cyclically pass. In this way, lateral zones are partially folded onto main portion of pack.
As they slide onto second portions of rails, lateral zones are completely folded onto main portion of pack.
Though efficient, packaging machines of the above type leave room for improvement.
In particular, the central portion of the top fin is folded by a relative movable plate whereas the lateral portions are folded by the wheels and the rails.
Furthermore, the top end is folded onto the main portion of the relative packs in two subsequent steps. The first step is carried out by the relative movable plate and by the wheels and the first portions of rails while the second step is carried out by the second portions of rails.
A need is felt within the industry to render as precise and repeatable as possible, the folding of the top end fin onto the main portion of packs.
Furthermore, in the known folding unit, the second folding means completely fold the top flaps of the top end onto the main portion.
The top flaps are therefore pressed onto the main portion of the formed package by a pressure device which is arranged downstream from the first folding means and is shown in EP-B-0887261 and which substantially comprises three endless belts fixed relative to the conveyor chain.
Accordingly, the forming of the packages is substantially controlled by the pressure device which defines with the chain conveyor a forming passage having a constant rectangular section, and defining the outer contour of the finished packages.
However, a wide range of modified package shapes has been developed which are different from the parallelepiped package.
In particular, packages with a slightly rounded or an octagonal cross section have been developed.
For these packages, the Applicant has found that the forming operation may require some adjustments.
This is mainly due to the fact that the forming passage must be, in this case, polygonal whereas the endless belts have substantially flat surfaces cooperating with the folded package.